A typical cradle swash plate type piston pump is configured such that: a rear surface of a swash plate projects in a circular-arc shape; a casing or a swash plate support is formed to have a circular-arc support surface to support the circular-arc rear surface of the swash plate; and a tilt angle of the swash plate with respect to a rotating shaft changes by tilting the swash plate while introducing lubricating oil to the support surface, thereby adjusting the amount of hydraulic oil discharged (see Japanese Laid-Open Patent Application Publication Hei 11-50951 for example). Specifically, this type of piston pump includes a plurality of pistons arranged in a circumferential direction in a cylinder block disposed in the casing. When the cylinder block rotates by rotation of the rotating shaft, the pistons reciprocate while tip end portions thereof are guided along the swash plate, thereby sucking/discharging the hydraulic oil. At this time, the increase in the tilt angle of the swash plate increases the stroke of the piston, thereby increasing the amount of hydraulic oil discharged, whereas the decrease in the tilt angle of the swash plate decreases the stroke of the piston, thereby decreasing the amount of hydraulic oil discharged.
In the foregoing swash plate type piston pump, since a reaction force applied by the hydraulic oil to the pistons when the pistons move back into the cylinder block and discharge the hydraulic oil acts on the swash plate, a surface pressure between the swash plate and the swash plate support becomes very high. Therefore, a lubricating oil film at an interface between the swash plate and the swash plate support tends to run out. On this account, slide surfaces of the swash plate and the swash plate support require seizing resistance and abrasion resistance. Conventionally, the seizing resistance and the abrasion resistance are given to the swash plate and the swash plate support, made of cast iron, by gas nitrocarburizing which causes nitrogen to diffusively intrude into the swash plate and the swash plate support to harden those surfaces.
(A piston pump sucks/discharges the hydraulic oil using the pistons by utilizing, as an input, a driving force applied to the rotating shaft. A piston motor has the same basic configuration as the piston pump except that the piston motor outputs the driving force of the rotating shaft by utilizing, as an input, inflowing/outflowing pressure oil. Therefore, the piston pump is referred to as a piston pump motor in the present description.)